Phase locked loops are widely used in electronic systems. For example, phase locked loops are widely used in communication systems including but not limited to radio frequency communications systems such as radiotelephones. FIG. 1 is a block diagram illustrating a conventional phase locked loop.
Referring now to FIG. 1, a conventional phase locked loop 100 includes a controlled oscillator 102 such as a voltage controlled oscillator (VCO) that is responsive to a control signal 104, to generate an output signal 106, the frequency of which is a function of the control signal. A sinusoidal phase detector 108, also referred to as a multiplier, is responsive to a reference frequency input signal 112 and to the output signal 106 to produce an error signal 114. A loop filter 116 filters the error signal 114, to thereby produce the control signal 104 that is provided to the voltage controlled oscillator 102. The loop filter is designed to provide stability of the phase locked loop. The phase locked loop may also be responsive to an offset frequency signal 118 using a frequency converter 120 that is responsive to the output signal 106 and to the offset frequency signal 118. The output of the frequency converter may be filtered by a bandpass filter 122. When an offset frequency is used, the phase detector 108 is responsive to the output signal 106', the frequency of which is offset from the frequency of the VCO output signal 106 by the frequency of the offset frequency signal 118. The design and operation of the phase locked loop 100 and the individual components thereof are well known to those having skill in the art and need not be described further herein.
As the performance of electronic systems continue to improve, it is desirable to obtain improved performance from the phase locked loop. As is well known to those having skill in the art, since the phase detector works in a linear region, amplitude fluctuations at the inputs thereof can produce phase detector gain variation and consequently phase locked loop bandwidth variations, and may result in undesired modulation of the voltage controlled oscillator 102. Accordingly, it is important to control the amplitude of the reference frequency input signal 112 that is provided to the sinusoidal phase detector 108.
One conventional technique to control the amplitude of the reference frequency input signal 112 is to include an automatic gain control circuit that is responsive to a phase locked loop input signal, to thereby generate a reference frequency input signal having a controlled amplitude. Unfortunately, the use of a separate automatic gain control circuit may unduly increase the complexity of the phase locked loop.
Another conventional technique is to place a gain control circuit within the phase locked loop, to thereby control the amplitude of the control signal 104 or of the error signal 114. Such a gain control within the loop may include a variable gain circuit. Unfortunately, the addition of an automatic gain control circuit within the phase locked loop may unduly degrade the noise performance and/or complexity of the phase locked loop.